The present invention relates to a method for fabricating a thin-film substrate, and a thin-film substrate fabricated by the method, and more particularly, to a method for fabricating a thin-film substrate for use in the fabrication of a microstructure, and so forth, applicable to a semiconductor device, a liquid crystal light modulator, or a MEMS (microelctromechanical system), and a structure of a thin-film substrate fabricated by the method described.
A silicon wafer, glass substrate, or quartz substrate has so far been in widespread use as a substrate for use in fabrication of a thin-film transistor, other thin-film devices, or a MEMS. It is more often than not that these substrates are formed to a thickness in a range of several hundred xcexcm to several mm.
These substrates which are further ground are used as an extra-thin substrate at times, and for example, a silicon wafer on the order of 10 xcexcm in thickness can be obtained from Virginia Semiconductor Inc. (1501 Powhaten Street, Fredericksburg, Va. 22401, USA), and so forth.
Further, there are cases where a film of a polymer such as polyimide, formed on a glass substrate by the spin cast method, is used as a thin-film substrate severalxcexcm or less in thickness.
Furthermore, there are also times when a thin-film of a metal oxide and so forth, formed directly on a silicon wafer or a glass substrate by the sputtering method or the chemical vapor deposition (CVD) method, is used as a thin-film substrate.
There has been adopted another approach wherein supports are provided on a support base, and on top of the supports, a thin-film serving as a functional structure or as a substrate of the functional structure is formed.
As a conventional technique of constituting a structure so as to be spaced away from the support base, there is available a method for fabricating a movable mirror of a spatial light modulator as disclosed in U.S. Pat. No. 4,956,619.
The conventional method for fabricating the structure is described hereinafter with reference to plan views of FIGS. 44 to 47, and the steps of the method for fabricating the structure are described with reference to cross-sectional views of FIGS. 48 to 51, respectively taken on line 48xe2x80x9448 in FIG. 44.
As shown in FIG. 48, a spacer layer 203 a portion of which is to serve as a sacrifice layer is first applied to the top of a substrate 201 by the spin coater method, and a metal layer 205 is formed on top of the spacer layer 203. The metal layer 205 is formed of an aluminum alloy with copper added thereto in order to enable it to function as a reflecting film.
Further, the metal layer 205 is patterned by photolithography in order to form a flap 101 which is to become a pixel of a light modulator from the metal layer 205. Then, as shown in FIGS. 44 and 48, a plasma etch access gap 105, and plasma etch access holes 103 for forming the flap 101 are provided in the metal layer 205.
Subsequently, a cavity region 107 shown in FIG. 49 is formed by plasma etching based mainly on oxygen. FIG. 45 is a plan view of the flap with the cavity region 107 formed therein.
By introducing active species through the plasma etch access holes 103 and the plasma etch access gap 105, positive photoresist of the spacer layer 203 is isotropically etched, thereby forming the cavity region 107.
As etching proceeds further, the cavity region 107 expands as shown in FIGS. 46 and 50. Thereafter, as shown in FIGS. 47 and 51, etching is continued until the formation of a structure wherein the flap 101 is floatingly spaced away from the substrate 201 to be retained by the spacer layer 203. As a result of such processing as described above, the structure can be formed wherein the flap 101 is floatingly spaced away from the substrate 201.
As another method for implementing a similar structure for a light modulator, there is an example as disclosed in U.S. Pat. No. 4,592,628.
With this method, silicon (Si) is used for a spacer layer which is to serve as a sacrifice layer, and silica (SiO2) obtained by oxidizing silicon is used for a thin-film structure, removing portions of the spacer layer by the wet etching method wherein an etchant capable of selectively etching silicon only such as, for example, pyrocatechol ethylenediamine is introduced through openings.
Further, as another method wherein supports are provided on a support base, a sacrifice layer is made use of in order to form a thin-film serving as a functional structure or a substrate of the functional structure on top of the supports, and the sacrifice layer is removed by etching through openings, there is disclosed a method for removing the sacrifice layer by a vapor phase etching method in xe2x80x9cJP, 10-107339, Axe2x80x9d.
However, the conventional silicon wafer, glass substrate, or quartz substrate for use in the fabrication of the thin-film transistor, other thin-film devices or MEMS has often been on the order of several hundred xcexcm to several mm in thickness.
Accordingly, in the case of using these substrates, for example, when applying heat treatment to a device or structure formed thereon, there has arisen a problem in that heat treatment can not be effectively applied thereto because an object being for heat treated on these substrates is often made up of a thin film of several xcexcm or less in thickness, and heat is dissipated from the object for being heat treated to these substrates although heat capacity of the former is very small. Further, in the case of forming a thin metal film for forming wiring, another problem has been encountered in that the thin metal film needs to be insulated and retained by a dielectric film having dielectric constant at least several times greater than that of air or vacuum, and consequently, a large floating capacity is added to wiring in a high-frequency circuit.
The problems described above also occur to a substrate wherein an inorganic or organic thin film is formed directly on the silicon wafer, glass substrate, or so forth.
Further, as a particularly thin substrate, there is available a silicon wafer, for example, on the order of 10 xcexcm in thickness, obtained by further grinding these substrates, but the silicon wafer is weak in mechanical strength, requiring special care in handling and transportation, and it is physically difficult to render the same further thinner, which limits a range of application thereof.
Accordingly, as a method for making the most of the function of a thin film, it is conceivable to make use of a process wherein supports are provided on a support base, and on top of the supports a thin-film serving as a functional structure or as a substrate of the functional structure is formed as practiced in a method for fabricating the MEMS having a space structure such as, for example, a DMD (digital micromirror device).
With the process proposed so far, however, there has been adopted a method wherein a thin film is formed on top of a spacer layer which is to serve as a sacrifice layer, and the sacrifice layer is removed by plasma etching, wet etching, or gas phase etching through openings provided in the thin film.
In such a case, since etching of the sacrifice layer proceeds mainly only in the traverse direction through the openings, a multitude of the openings are required. An area of the sacrifice layer, normally removable by one of the openings, is practically on the order of several thousand xcexcm2 owing to a low etching rate in the traverse direction.
There have recently been seen tendencies for a semiconductor integrated circuit (IC), a circuit board, and further, a MEMS, and so forth, formed on a silicon wafer, glass substrate, or so forth, to increase in scale and size, and to become higher in density, and consequently, a thin film having a relatively large area is often used as a substrate. However, a thin film provided with a multitude of openings will become unsuitable for such application.
It is therefore an object of the invention to enable a thin film substrate having an area even as large as several mm2 or 10 mm2 or more to be formed in such a way as to be floatingly spaced away from, and retained by a support base.
To this end, the invention provides a method for fabricating a thin-film substrate, wherein the thin-film substrate is fabricated on top of supports arrayed on a support base so as to be spaced away from the support base, said method comprising the steps of:
forming the supports in a predetermined shape on the support base;
forming a sacrifice layer made of a resin material on the support base;
planarizing the sacrifice layer so as to expose the top of the respective supports;
forming a thin-film substrate on top of the sacrifice layer as planarized; and
removing the sacrifice layer by plasma selective etching through the intermediary of the thin-film substrate.
The method for fabricating the thin-film substrate described above may further comprise a step of forming an opening functioning as a discharge port of a volatile gas evolved as a result of the plasma selective etching in the thin-film substrate formed on top of the sacrifice layer, between the step of forming the thin-film substrate and the step of removing the sacrifice layer.
Further, the supports are preferably formed in the shape of a pillar such as a cylinder, prism, or so forth, or in a shape of a wall such as a cuboid or so forth.
If the supports are formed of an electrically conductive material, this can promote etching in the step of removing the sacrifice layer by the plasma selective etching.
The thin-film substrate may be formed of an insulating material.
In the step of forming the supports in the predetermined shape on the support base, it is preferable to form a metal film on the support base, and to form the supports in the predetermined shape by applying photolithography and etching treatment to the metal film.
In the step of removing the sacrifice layer by the plasma selective etching through the intermediary of the thin-film substrate, the sacrifice layer is preferably removed by plasma selective etching based mainly on a reactive gas containing oxygen or oxygen atoms through the intermediary of the thin-film substrate. A total pressure applied at this time is preferably 100 Pa or more.
Further, the sacrifice layer is preferably formed of material containing acrylic resin as the main constituent thereof or a photoresist material.
Another preferred embodiment of a method for fabricating a thin-film substrate according to the invention comprises the steps of:
forming a metal film on the support base by the sputtering method;
forming the supports in a predetermined shape by applying photolithography and etching treatment to the metal film;
forming a sacrifice layer by applying a resin material containing acrylic resin as the main constituent onto the support base by means of the spin coater method;
planarizing the sacrifice layer by etching the same through a plasma treatment so as to expose the top of the respective supports;
forming the thin-film substrate made of an insulating material on top of the sacrifice layer as planarized; and
removing the sacrifice layer by plasma selective etching based mainly on a reactive gas containing oxygen or oxygen atoms with a total pressure at 100 Pa or more through the intermediary of the thin-film substrate.
The method for fabricating the thin-film substrate described above may also further comprise a step of forming an opening functioning as a discharge port of a volatile gas evolved as a result of the plasma selective etching in the thin-film substrate formed on top of the sacrifice layer, between the step of forming the thin-film substrate and the step of removing the sacrifice layer.
A base formed of material containing silica (SiO2) as the main constituent thereof or material containing silicon (Si) as the main constituent thereof is preferably used for the support base.
A support base formed of material containing silica (SiO2) or silicon (Si) as the main constituent thereof with a metal oxide thin-film formed thereon may be used for the support base.
In this case as well, the supports are preferably formed in the shape of a pillar, or in the shape of a wall.
The thin-film substrate is preferably formed of material containing tantalum oxide (TaOx) as the main constituent thereof, or material containing silicon oxide (SiOx) as the main constituent thereof.
Further, in the step of forming the metal film on the support base, the metal film is preferably formed of material containing molybdenum (Mo) as the main constituent thereof.
Further, in accordance with another aspect of the invention, a method for fabricating a thin-film substrate may comprise steps similar to those previously described, whereby the supports in a predetermined shape and a sacrifice layer are formed on a support base, the sacrifice layer is planarized so as to expose the top of the respective supports, subsequently, a porous film is formed on the sacrifice layer as planarized, and the sacrifice layer is removed by plasma selective etching through the intermediary of the porous film before forming the thin-film substrate on the porous film.
Because the plasma selective etching is carried out through the intermediary the porous film according to this method for fabricating the thin-film substrate, an etching rate can be increased, time required for the removal of the sacrifice layer can be shortened, and the thin-film substrate can be supported with greater certainty.
Further, the invention provides a thin-film substrate fabricated by each of the previously described method for fabricating the thin-film substrate.
With the methods for fabricating the thin-film substrate according to the invention, on the support base to be a supporting member for the thin-film substrate, the supports for supporting the thin-film substrate, and the sacrifice layer for forming the thin-film substrate so as to be floatingly spaced away from the support base are formed, and the sacrifice layer is planarized. The thin-film substrate is formed on top of the sacrifice layer, and only the sacrifice layer is selectively removed by carrying out the plasma selective etching on predetermined conditions through the intermediary of the thin-film substrate.
At this point in time, since the plasma selective etching of the sacrifice layer proceeds through the thin-film substrate, there is no particular need of providing openings, and even if the openings are provided, only a few thereof will suffice. Accordingly, there is few limitations to an area of the thin-film substrate to be formed, and the thin-film substrate having a large area, for example, a several ten mm square or more, can be formed so as to be floatingly spaced away from the support base by virtue of the supports.
Although there remain many obscure points to a detailed mechanism of the plasma selective etching of the sacrifice layer proceeding through the thin-film substrate, the inventors are of the following opinion.
In the case of using an organic film such as positive photoresist, acrylic resin, or so forth for a constituent material of the sacrifice layer, the plasma selective etching can be implemented by use of a gas containing oxygen. For example, an assumption is made that the sacrifice layer composed of acrylic resin is etched by oxygen as an etching gas.
If etching by excitation of an oxygen gas is carried out in an etching chamber having a parallel plate type electrode structure with the use of a 13.56 MHz RF power supply source, etching proceeds isotropically under pressure at several hundred Pa. As active species of the etching at this time, oxygen ion, neutral atomic oxygen, and oxygen radical are conceivable.
The thin-film substrate made of an insulating material such as tantalum pentaoxide (Ta2O5), formed on top of the sacrifice layer by sputtering, is not etched by oxygen plasma, however, in the case where the thin-film substrate is of several hundred nm or less in thickness, upon etching thereof by oxygen plasma, a portion of active species in the oxygen plasma is allowed to pass through the thin-film substrate because it is sufficiently thin, and the portion of the active species, passed therethrough, will reach the sacrifice layer, thereby starting reaction with the outermost surface of resin.
As a result, reaction products such as CO2 gas, H2O gas are generated at the interface between the thin-film substrate and the resin. Transformation of the acrylic resin in a solid form into gas taking place at the interface will cause substantial expansion in volume, and pressure built up as a result acts as a driving force to discharge the reaction products to the outside of the thin-film substrate therethrough.
When the pressure reaches a state of equilibrium, the active species in the plasma are supplied to the surface of the resin through the thin-film substrate from the outside thereof, and thereby reaction occurs between the resin and the active species, so that the etching of the sacrifice layer proceeds. Small gaps developed at the interface between the thin-film substrate and the resin gradually expand, eventually forming a space.
Further, the inventors have discovered that in the case of forming the supports for retaining the thin-film substrate from an electrically conductive material such as, for example, molybdenum (Mo), the etching proceeds at a high rate, particularly from the vicinity of the respective supports.
In view of the fact that such a phenomenon is not observed in the case of using an insulating material for the supports, it is presumed that the supports as electrically charged during plasma discharge play a role of increasing concentration of the active species in the vicinity of the respective supports due to some action thereof, and a supply rate of the active species through the thin-film substrate, in the vicinity of the respective supports, is accelerated, thereby causing the phenomenon to occur.
Consequently, an etching rate in the case of using an electrically conductive material for the supports is substantially increased as compared with the case of using an insulating material for the supports. Hence, the former is suitable particularly in the case of fabricating a thin-film substrate having a large area.
Further, since the etching of the sacrifice layer is implemented through the thin-film substrate, the thinner the thickness of the thin-film substrate, the higher an etching rate becomes.
Furthermore, the higher a pressure of a gas introduced, the higher an etching rate becomes. For example, in the case of employing the supports having insulating properties, the etching through the thin-film substrate hardly proceeds at a pressure not higher than about 70 Pa or less, however, the etching rate is increased substantially in proportion to a pressure level in a range of about one hundred Pa to several kPa.